RFR: 8287157: Improve documentation in G1Policy::next_gc_should_be_mixed()

[Thomas Schatzl]

Hi all,

  can I have reviews for some minor cleanup in `G1Policy::next_gc_should_be_mixed()`, removing an unused parameter and arguably improving the comment inside.

Testing: local compilation.

Thanks,
  Thomas

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Commit messages:
 - 8287157: Improve documentation in G1Policy::next_gc_should_be_mixed()

Changes: https://git.openjdk.java.net/jdk/pull/8844/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8844&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8287157
  Stats: 11 lines in 2 files changed: 1 ins; 3 del; 7 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8844.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8844/head:pull/8844

PR: https://git.openjdk.java.net/jdk/pull/8844